Anders Mikkelsen e-mail: anders.mikkelsen@sljus.lu.se ... Lecture 6 2014.pdf · Imaging with SPM...

~100x100nm

~10x10nm

~1000x1000nm ~100000x100000nm

Anders Mikkelsen

e-mail: anders.mikkelsen@sljus.lu.se

Synchrotron Radiation Research, Fysicum

www.sljus.lu.se

Experimental methods are based on:

Atomic Force Microscope

• Scanning Tunneling Microscopy (STM)

• Scanning Tunneling Spectroscopy (STS)

• Scanning Tunneling Luminesence (STL)

• Atomic Force Microscopy (AFM)

• Magnetic Force Microscopy (MFM)

• Electrostatic Force Microscopy (EFM)

• Capacitance Microscopy (CM)

• Scanning Nearfield Optical Microscopy(SNOM)

• ...

Some Scanning Probe Techniques

• Scanning Tunneling Microscopy (STM)

Resolve individual atoms, measure electrical properties, induce photo luminesence.

Only conducting samples.

• Atomic Force Microscopy (AFM)

Resolution limit ~STM, but more difficult to achieve.

Any sample type.

Some Scanning Probe Techniques

Imaging with SPM

Red blood cells DNA on lipid-bilayer Butterfly wing

Carbon nanotube III-V nanowire Magnetic domains

Nanomanipulation

and electron density

waves by STM:

Quantum Corrals (Don Eigler IBM)

Condensed matter & Chemistry

applications of SPM

• Range: 10x10nm – 10x10mm

• Resolution: ~0.01nm

• Typical environment: UHV

• Samples: Conducting

• Topography

• Geometric structure

• Electronic structure

• Vibrational structure

• Magnetic structure

• Manipulate atoms / molecules / nanostructures

• Range 10x10nm – 100x100mm

• Resolution: 1 – 0.01nm

• Typical environment: Air/Liquid

• Samples: All types

• Topography

• Geometric structure

• Friction

• Adhesion

• Hardness

• Manipulate molecules / nanostructures

STM AFM

• Scanning Tunneling Microscopy (STM)

• Scanning Tunneling Spectroscopy (STS)

• Scanning Tunneling Luminesence (STL)

• Atomic Force Microscopy (AFM)

• Magnetic Force Microscopy (MFM)

• Capacitance Microscopy (CM)

• Scanning Nearfield Optical Microscopy(SNOM)

• ...

Some Scanning Probe Techniques

The Scanning Tunneling Microscope

Two ways of forming the STM image:

a) is almost always used!

The expermental setup:

a) I constant, feedback loop active,

Z variation measured

b) Z constant, feedback loop idle,

I variation measured

STM systems at Fysicum

• Omicron STM1 (in C163)

High resolution

• Omicron XA STM + PEEM (in C162)

20mm scan range, low noise, precise positioning

• JEOL STM (in C161)

• Exploratory

• Omicron VT STM (in C164)

Variable Temperature 15-1500K

Sources available:

• Ga, In and As MBE sources

• Atomic Hydrogen source

• Oxygen, Ammonia

from 1-10mbar to 1bar

Experimental issues STM tips made by

electrochemical etching STM stage with magnetic damping

Tripod STM with magnetic damping

STM stage –side view

A) Tripod scanner

B) Sample stage

C) Magnetic damping

D) Current amplifier

STM top view

Scanner design

SPM scanners are made from a piezoelectric

material PZT, Lead Zirconium Titanate.

PZT expands and contracts proportionally to an

applied voltage.

In tripod design three piezos

are used to move in XYZ

Piezoelectric scanner:

• based on images of known surfaces calibrate the

piezoelectric response to convert [Volts] to [nm]

• the response is continuous down to atomic lengths

scales

• unfortunately, the response is highly non linear

(hysteresis)

Scanner designs

Precise coarse movement with piezos:

Piezo tube can also be used as scanner

Clean surfaces

• Sputtering the surface with Ar-ion bombardment (to remove contamination on the surface)

• After Sputtering: Annealing to high temperatures to smooth the surface

• Chemical cleaning by O2 treatment at higher temperature (burns away the carbon)

• Or by H2 treatment at high temperatures

Alternatively:

• evaporate well-ordered films on a substrate

(Attard 1.9)

Watching Surface Quality:

Surface Crystallography: LEED (low energy electron diffraction)

Chemical Surface composition: AES, XPS

Cleaning surfaces Keeping them clean

Pressure (mbar)

1000

100

10-3

10

10-11

10-13

10-17

1

10-1

10-2

10-4

10-5

10-6

10-7

10-8

10-9

10-10

10-12

10-14

10-15

10-16

8 km 10-3 km

30 km

90 km

278-460 km

The pressure gap

Vacuum Vacuum is measured in units of pressure. The SI unit of pressure is the pascal (symbol

Pa), but vacuum is usually measured in torrs (symbol Torr), named for Torricelli, an

early Italian physicist (1608 - 1647). A torr is equal to the displacement of a

millimeter of mercury (mmHg) in a manometer with 1 torr equaling 133.3223684

pascals above absolute zero pressure.

Electron tunneling in 1D from I to III

Time independent Schrödinger equation: 1D model of tunneling from

one side (I) to the other (III)

Y(x) oscillate in region I and III

Y(x) decay exponentially in region II

outgoing current density

incomming current density T=

T~exp(-2ka), k2~(V0-E)

T falls of exponentially with barrier width

Sensitivity of STM

Tunneling current is proportional to exp(-2Kd) ; K=(2mf)/h

For average work function f of ~4eV, K=1Å-1

=>

Changing d ~0.1 nm will change current by an order of magnitude!

Typical currents: 0.1-1nA Typical voltages: 0.1-4V

0.001

0.002

0.005

0.01

0.21 0.23 0.25 0.27 0.29 0.31 0.33 0.35 0.37 0.39

Distance between tip and sample, d in nm

log(N

om

aliz

ed T

unn

elin

g c

urr

ent)

Mophology of samples

What are we imaging at the atomic

scale?

Local Density of States

• Density of state (DOS) means the "number of states at a

particular energy level", i.e. the distribution of states over

energy.

dE

dNE E)(

• LDOS (x,y,E)

gives the density

of electrons of a

certain energy at

that particular

spatial location.

Electron tunneling in 1D from I to III

Time independent Schrödinger equation: 1D model of tunneling from

one side (I) to the other (III)

Y(x) oscillate in region I and III

Y(x) decay exponentially in region II

outgoing current density

incomming current density T=

T~exp(-2ka), k2~(V0-E)

T falls of exponentially with barrier width

Bardeen’s tunneling current

formalism

Basic assumptions:

Fermi’s golden rule + Coupling between surface & STM tip, is very low

=>

Treat tunneling as a slight perturbation of the electronic structure.

Then the tunneling current can be computed as the overlap of

wave functions of the sample and the tip

Tunneling current:

Fermi function Tunneling matrix elements

yt/s ; t/s are wave functions and density of states of tip/sample

Further simplifications

(Tersoff & Hamann)

Further assume an S-wave tip with

constant density of states:

Assume low voltage:

This means:

I depends on the local density of states of the sample near the fermi level

which can be calculated by standard ab-initio theoretical methods!

S-wave tip

s is evaluated at the center of curvature

of the tip

Energy levels involved in

tunneling

(b)

(c)

(d)

(a)

Density of states

Energy level diagram for sample

and tip:

(a) Independent sample and tip.

(b) Sample and tip at equilibrium,

separated by a small vacuum

gap.

(c) Positive sample bias: electron

tunnel from the tip to the sample.

(d) Negative sample bias: electron

tunnel from the sample into the

tip.

What are we imaging in STM?

Some general concepts can also be derived:

Metals:

High density of states at atoms => atoms appear as bright protrusions

Insulators:

No conduction possible => we crash

Semiconductors and thin oxides:

Complex electronic structure at fermi level => be careful!

e-

d d

Positive Bias Positive Bias Negative Bias

Negative Bias

FS

FT 1eV

Measuring empty or filled states

by bias switching

Potential Barrier Potential Barrier

Tip Sample Sample Tip

Filled Electronic Valence Band States

Filled state imaging Empty state imaging

e- FS

FT

GaAs(110) – an example of different

filled / empty state imaging

High density of states in conduction band

above Ga atoms

High density of states in valence band

above As atoms

We can change bias to image different

species.

LEED pattern

Surface Crystallography in the Virtual Lab

• common practice: trial and error modeling

– set up models and compare with experiment

• STM

• vibrational spectroscopy (EELS)

• core-level binding energies

• LEED

– set up a number of reasonable models and calculate their energy the one with the lowest energy is most stable (“ab initio” thermodynamics)

• Ab-initio theoretical method assumptions

– Quantum mechanics with a few simplifying approximations

– Basic structural model (density, type and symmetry of atoms)

– Thermodynamc equilbrium

– S-wave STM tip

Favorable model from theory

7 Pd5O4 cells on undistorted 3 layer substrate (6x8=48 Pd atoms per layer)

Three fold Oxygen

Four fold Oxygen

4 fold Pd atoms are above Pd-Pd rows

Comparison of STM images

Experimental STM image

Theoretically simulated STM image

Tip influences sample...

A recent example..

Wires are deposited via a dry deposition

method onto a flat substrate outside

vacuum.

Subsequently the sample is inserted into

the STM vacuum chamber and annealed to

~350-400C in the presence of atomic

hydrogen.

Note:

Anneal temperature is sufficiently low that wires and devices should not melt!

High resolution STM imaging

of wurtzite InAs/InP nanowires

Overview STM images of InAs

nanowires

STM image of InAs NW STM image of wire bundles

STM traces

Corrugation in STM will be smaller on bundles!

Crystal engineered nanowires with many

different structures STM imaging with atomic resolution possible along micron long wires!

Twinned ZB{110} Twinned ZB{110}

WZ{11-20} WZ{10-10}

Twinned ZB{110} Twinned ZB{110} Twin super lattice

WZ{11-20} WZ{11-20} WZ{10-10}

200 nm

Scanning Electron Microscopy

Scanning Tunneling Microscopy(STM)

ZB{110} WZ{11-20} WZ {10-10}

Atomic scale structure imaged on all different

facets of the same wire

Twinned ZB{110} Twinned ZB{110} Twin super lattice

WZ{11-20} WZ{11-20} WZ{10-10}

Imaging down into trenches of a twin plane

superlattice surfaces with atomic resolution

Twinned ZB{110} Twinned ZB{110} Twin super lattice

WZ{11-20} WZ{11-20} WZ{10-10}

100x100nm2

10nm

{111}B

{111}A

2nm

Wurtzite {11-20}

Zincblende {110}

Spectroscopy across the Wurtzite / Zincblende

interface

Twinned ZB{110} Twinned ZB{110} Twin super lattice

WZ{11-20} WZ{11-20} WZ{10-10}

• Scanning Tunneling Microscopy (STM)

• Scanning Tunneling Spectroscopy (STS)

• Scanning Tunneling Luminesence (STL)

• Atomic Force Microscopy (AFM)

• Magnetic Force Microscopy (MFM)

• Capacitance Microscopy (CM)

• Scanning Nearfield Optical Microscopy(SNOM)

• ...

Some Scanning Probe Techniques

STM on an AFM

Original AFM was based on a STM as

transducer.

Atomic Force Microscopy (AFM)

or

Scanning Force Microscopy (SFM)

AFM probe surface with a sharp tip

Forces between tip and cantilever deflects

tip.

A detector measures the deflection

Two basic operational modes:

Contact mode

Non-contact mode

AFM vs STM

Kubo and Nozoye. Physical

Review Letters, 86(9), 1801–1804, 2001

STM NC-AFM

Low temperature STM and NC-AFM

images on graphite

PNAS, 100(22), 12539–12542, 2003

Forces: simple view

E. Meyer, H. J. Hug, R. Bennewitz; “Scanning Probe Microscopy: The Lab on a Tip”

(some discussion on forces also in Attard 1.13)

Forces between tip and surface

during AFM

Detecting Cantilever Deflection

Photodiodes

Laser Diode

Mirror

Cantilever

and Tip Feedback

X-Y-P PiezoScanner

Image

Common detection schemes Quartz resonator

AFM detection system Quite simmilar to our STM experimental setup!

Some of our AFMs

EduScope AFM

Specifications

X-Y scanning range 50x50 μm2

Z scanning range 6 μm

Resolution AFM mode

1-10nm

JHK NANOWIZARD II Bio

AFM

Atomic lattice resolution on inverted

microscope (< 0.055 nm RMS z noise level)

100 x 100 x 15 µm3 scan range

All standard AFM modes – contact mode,

Lateral Force Microscopy (LFM), AC modes, force modulation,

force spectroscopy, force mapping, nanomanipulation,

nanolithography, etc.

Omicron Q-plus sensor AFM/STM

AFM image of biomolecules

Human chromosone 190 nm-long DNA strands

AFM image of blood clotting

• Polymerization of fibrin catalyzed by the presence of

thrombin

• Mechanism for blood clotting

B. Drake et al., Science 243, 1586 (1989)

High resolution AFM of supramolecular

assembly of the photosynthetic

complexes in native membranes

Scheuring, S., Sturgis, J.N., 2005. Chromatic adaptation of photosynthetic

membranes. Science 309, 484–487.

AFM tips

Three common types of AFM tips

Commercial, from silicon or silicon nitride:

-Standard chip size: 1.6 x 3.6 x 0.4 mm (more than

1000 probes from aSiwafer)

-High reflective Au coating (reflective property is 3 times

better in comparison with uncoated cantilevers)

-Typical curvature radius of a tip: 10 nm

-Cantilever length: 100 -200 μm

-Cantilever width: 10 -40 μm

-Cantilever thickness: 0,3 -2 μm

-Available for NONCONTACT, SEMICONTACT

and CONTACT modes

-Triangular (V shaped) and rectangular

cantilevers

-Available with conductive TiN, W2C, Pt, Au and

magnetic Co coatings

Different cantilever shapes

Tip Artifacts

Double tip

Overestimate object size

Underestimate object size

Tip shape imaged

AFM image with Double tip

Tip artifacts

AFM Modes

E. Meyer, H. J. Hug, R. Bennewitz; “Scanning Probe Microscopy: The Lab on a Tip”

By: B. Resel

Non contact AFM

Detection scheme:

Static deflection

Dynamic Mode: Amplitude change

Frequency shift

High resolution AFM in non contact mode

(frequency modulated)

Si(111)-(7x7) AFM vs. STM

AFM STM

(A)The tip is approached to the surface until contact occurs.

(1) Retracting the cantilever stretches the connection of the single biomolecule to the surfaces.

When the force reaches the unbinding force of the complex, the biological interaction is

ruptured and

(2) The cantilever is available for a new force distance curve.

(B) Loading rate dependence of the unbinding forces of the avidin-biotin system under

physiological conditions

( Single Mol. 1 (2000) 285.)

Force spectroscopy

AFM - STM overview

STM AFM

• Range: 10x10nm – 10x10mm

• Resolution: ~0.01nm

• Typical environment: UHV

• Samples: Conducting

• Topography

• Geometric structure

• Electronic structure

• Vibrational structure

• Magnetic structure

• Manipulate atoms / molecules / nanostructures

• Range 10x10nm – 100x100mm

• Resolution: 1 – 0.01nm

• Typical environment: Air/Liquid

• Samples: All types

• Topography

• Geometric structure

• Friction

• Adhesion

• Hardness

• Manipulate molecules / nanostructures

Science applications of SPM

• Atomic scale structure of surfaces and

nanostructures

• Electrical properties on the nanometer scale

• Molecular bonding and structure of individual

molecules

• Morphology during chemical reactions

• Magnetic structure of very small objects

• Electronic, vibrational and geometrical structure

correlation down to the atomic scale

• Diffusion and growth studied directly on the

atomic scale

Technology applications

• Texture / roughness / topography of

materials and lithographic structures

• Electrical vs structural properties of chip

technology

• Magnetic vs structural properties of hard

disk disks

Some examples of SPM applications in CM technology

A cross-sectional scanning tunneling microscopy study of a quantum dot infrared photo-detector structure (Lund + Acreo AB)

Topography

MFM

Images of Over written Track on a Hard Disk TappingMode AFM phase and amplitude images of Celgard 2400

tape (membrane for Lithium batteries)

• Scanning Tunneling Microscopy (STM)

• Scanning Tunneling Spectroscopy (STS)

• Scanning Tunneling Luminesence (STL)

• Atomic Force Microscopy (AFM)

• Electrostatic Force Microscopy (EFM)

• Magnetic Force Microscopy (MFM)

• Capacitance Microscopy (CM)

• Scanning Nearfield Optical Microscopy(SNOM)

• ...

Some (more) Scanning Probe

Techniques

In situ AFM/STM/STS on NW devices combined AFM / STM / STS and conductivity measurements on

individually contacted and biased nanowire heterostructures

• interplay between atomic structure & electrical properties

• local changes of charge distribution upon external biasing

• influence of surface on conductivity

• AFM/STM QPlus sensor

• sample holder with four

separate electrical contacts

Vtip =

2 V

Vtip =

- 2 V

ISD ISD AFM

0.2 µm

Scanning gate microscopy

InAs GaSb contact

STM images on

top of NW

showing height AFM images, 2.6 x 0.5 µm

VSD = - 0.1 V ISD

biased STM tip

as local gate:

• strong gating

dependence of

the InAs part

• hardly any effect

at the GaSb part

AFM image, 3.5 µm x 3 µm

MFM: Magnetic Force Microscope AFM with magnetic probe

magnetic tip

laser photodiode

piezo-element

Magnetic Force Microscopy (MFM)

•Special probes are used for MFM. These are magnetically sensitized by

coating with a ferromagnetic material.

•The tip is oscillated 10’s to 100’s of nm above the surface

•Gradients in the magnetic forces on the tip shift the resonant frequency of the

cantilever .

•Monitoring this shift, or related changes in oscillation amplitude or phase,

produces a magnetic force image.

•Many applications for data storage technology

MFM of cobalt islands

m = -1 m = 0 m = 1

MFM image 100x140x20 nm Co particles

(square lattice; s=300 nm)

0 2.5 mm

0

1.0

degrees

Array of 80x140x18 nm Co nanostructures

200 nm

Electrostatic Force

Microscopy

EFM maps locally charged domains on the sample surface.

EFM can map the electrostatic fields of a electronic circuit as the device is turned on and off.

Electrostatic force microscopy (EFM) applies a

voltage between the tip and the sample while the

cantilever hovers above the surface, not touching

it. The cantilever deflects when it scans over

static charges.

Some material from:

• Dawn Bonnell Scanning Probe Microscopy

and Spectroscopy

Surface vs Nanowire

Surface Science, Invited Prospective 607, (2012) 97

Nanostructure surface studies at the NmC

Nanostructures

Growth substrates